Prime Minister Margaret Thatcher on Friday called her third straight general election victory her "most fantastic triumph," but she told her supporters to put the celebration behind them and get on with their work.

"We haven't got a lot of time to just sit around," she said. "We must get on. We have a lot to do."

Final returns from Thursday's election showed that her Conservative Party had been retained in power for five more years with a majority of 102 seats in Parliament--less than the 144-seat majority the Conservatives had in the last Parliament but far greater than they had expected in the face of a strong campaign by the Labor Party.

The Conservatives will hold 376 of the 650 seats in the new House of Commons. Labor will have 229, an increase of 21, and the centrist Liberal-Social Democratic Alliance won 22 seats. Smaller parties took the remaining seats.

20th Century Mark

Thatcher is the first prime minister since the 1820s to win a third consecutive term, a feat that could enable her to remain in office for a total of 13 years. By January, she will have served 8 1/2 years, the longest term of any British prime minister in this century.

London's financial markets soared on the Conservative victory. The stock market was up 27 points and closed at an all-time high.

Thatcher indicated she would announce a minor reorganization of her Cabinet during the weekend and begin immediately to prepare her new government's program. It will be announced when Parliament convenes June 25.

The sense of urgency in Thatcher's remarks Friday, only slightly mellowed by the glow of victory, were typical of the drive she has shown over the past eight years in her effort to eradicate socialism and transform Britain into a nation of homeowners, stockholders and entrepreneurs.

Continued NATO Role

On the foreign front, her reelection preserves continuity among the West's leaders and ensures that Britain will continue to play a prominent role in the North Atlantic Treaty Organization.

The Labor Party had campaigned on a pledge to scrap Britain's nuclear deterrent and to demand the withdrawal of the U.S. nuclear forces that have been based in Britain for decades.

In Berlin, President Reagan said that he was pleased with the outcome of the election and that he had telephoned Thatcher--whom he referred to by her first name--for a "10-minute chat." Chancellor Helmut Kohl of West Germany said it was "good for the Atlantic Alliance."

On the strength of Thatcher's post-election comments, it seems clear that her energies will be devoted mainly to domestic affairs. She said her policies and her tactics have proved to be right for Britain and that she has no intention of changing them.

Her formula, she said, will be "sound financial policy operated by government, continue to cut (red tape) and then the people supply the enterprise."

Home Ownership Goal

She said that one of her first priorities will be to implement new legislation to increase home ownership and to shake up the country's crisis-ridden education system by imposing a basic national curriculum in primary and secondary schools and giving sweeping autonomy to individual schools.

Both programs, she said, will benefit troubled inner-city areas.

"The whole point about housing and education is to bring increasing opportunity to some of those very people in the inner cities who feel they are trapped," she said.

She said she also plans to unleash private enterprise in a program to revive pockets of urban decay.

Despite the size of the Conservative victory, the party was set back sharply in the north of England and in Scotland, where it will control only 10 of 72 seats. Defense Secretary George Younger survived by just 184 votes in Ayr, his Scottish district. In the large cities of Manchester, Liverpool and Glasgow, not a single Conservative was elected.

The result virtually split the country, with the Conservatives sweeping the more populous, more prosperous south but losing support in the north, where many contend that Thatcher's policies have exacerbated industrial decline.

Labor Party leader Neil Kinnock referred Friday to a "greater abyss of division," and some commentators expressed concern that the split could cause increased social tension.

Many observers believe that despite Labor's losses, Kinnock's strong campaign erased the possibility of Labor's being relegated to third place among the parties. Thus Kinnock is expected to strengthen his hold on the party and continue to put emphasis on moderate positions.

David Owen and David Steel, the leaders of the Liberal-Social Democratic Alliance, had hoped that 1987 would be the year in which theirs would become the No. 2 party. But the alliance, by winning only 22 seats, is thought to have been crushed.

Alliance Losses

Alliance stalwarts Roy Jenkins, a former Cabinet minister and European Economic Community chief, lost his Glasgow constituency, and Shirley Williams, also a former minister and an Alliance co-founder, lost in Cambridge.

Four non-whites won seats, putting Britons of black and Asian descent in Parliament for the first time since the 1920s. Thatcher was among 41 women elected to the Mother of Parliaments, a record; their number includes the first black woman lawmaker in British history, Diane Abbott.

Thatcher's victory led to speculation about how long she might be prime minister, and she did little to discourage it.

In answer to a question about whether she might still be prime minister at the turn of the century, she said: "Well, you never know. I may be here, I may be twanging a harp. We'll just have to see how things go."